A blog about the history, current state, and the future of the electric guitar and tube-based amplifier combo and how this de rigueur musical instrument that matured during the Rock N' Roll-era continues to both evolve and refuses to die.

Tuesday, September 3, 2013

Even though it was the preferred vacuum tube for Jazz
guitarists near the end of the 1960s due to its use in the 1966 Ampeg GS-12R guitar
amp, is the 7591 vacuum tube deserve the acclaim as the Jazz guitarists’ vacuum
tube?

By: Ringo Bones

Sometimes I wonder whatever compelled the Russian vacuum
tube manufacturer Sovtek – via their Electro Harmonix arm – to remanufacture
the 7591 pentode output power vacuum tube during the first few years of the 21st
Century given that it became virtually extinct around the time when Ronald
Reagan ruled the free world. Was the Fisher 500-C and Hermon Hosmer Scott’s
receivers a runaway success during the height of the Cold War? Or was it that
the 1966 Ampeg GS-12R amp, oft used by Jazz guitarist on this side of the Iron
Curtain near the end of the 1960s, was also a runaway success for “underground”
Jazz guitarists in the then Soviet Union and the rest of the then Eastern Block
countries? Anyway, both guitarists and hi-fi enthusiasts are very eternally
grateful for Sovtek / Electro Harmonix for remanufacturing the 7591 vacuum tube
– not just for the still functioning 1966 Ampeg GS-12R amps the world over but
also the still working Fisher and Scott receivers needing fresh 7591 vacuum
tubes around the world.

The 7591 gained credence as the “Jazz guitarists’ preferred
vacuum tube” probably around the time of the Reagan presidency where NOS and
even (gasp!!!) used but still functional 7591 vacuum tubes were desperately
scrounged by discriminating Jazz guitarists and owners of Fisher and Scott
receivers in desperate need of fresh – or at least working - 7591 tubes. And
the 1966 Ampeg GS-12 guitar amp gained a reputation as a clean Jazz guitarists’
amp when it was introduced in the late 1960s even though it can rock when
pressed.

During the virtual extinction of the 7591 vacuum tube in the
middle of the 1980s, many 1966 Ampeg GS-12 guitar amp owners – as well as
Fisher and HH Scott receiver owners were experimenting ways to shoehorn the 6V6
vacuum tube – in the Ampeg – and the 6L6 vacuum tube – in the Fisher and Scott
receivers with various degree of success. It might have involved serious “brain
surgery” in rearranging the wiring and the biasing resistors and coupling
capacitors of their Ampeg amps and Fisher and Scott receivers since the 6V6 and
6L6 vacuum tubes are beam tetrodes while the 7591 tube is a pentode. Though 6V6
vacuum tubes that had been “successfully” transplanted into a 1966 Ampeg GS-12R
guitar amp did manage to produce a very interesting tone that make them now
highly collectible, sadly, less skilled thermionic electronic tinkerers irreparably
damaged their Ampegs in such endeavors.

Historically, the 7591 pentode output vacuum tube was
introduced in 1958 and registered with the EIA in 1960 via Westinghouse’s
receiving tube factory in Bath, New York. Given that Dr. Harvey C. Rentschler
used to be the director of research for the Westinghouse Lamp Division, every
vacuum tube enthusiast is now wondering how instrumental Dr. Rentschler was in
the development of the famed 7591 pentode output vacuum tube. And by the way,
the 1962 Westinghouse 7591 vacuum tube is the best NOS type for this kind of
tube.